FIG. 1 is a cross-sectional view of a wiring substrate 100 according to a related art example. With reference to FIG. 1, the wiring substrate 100 includes a substrate body 110, a first insulation layer 120, a first conductive layer 130, a second conductive layer 140, a second insulation layer 150, a third insulation layer 160, a first external connection terminal 170, and a second external connection terminal 180.
In the wiring substrate 100, the substrate body 110 is used as a substrate for forming layers such as the first conductive layer 130 thereon. A trench 110x and a through-hole 110y communicating with the trench 110x are formed in the substrate body 110. The first insulation layer 120 is formed on the first and second surfaces 110a, 110b of the substrate body 110, an inner bottom plane and an inner side plane of the trench 110x, and an inner side plane of the through-hole 110y. 
The first conductive layer 130 is formed on the first insulation layer 120 that covers the inner bottom surface and the inner side surface of the trench 110x and the inner side surface of the through-hole 110y. The second conductive layer 140 is formed on the first conductive layer 130 fills the inside of the trench 110x and the through-hole 110y. The first conductive layer 130 is electrically connected to the second conductive layer 140.
The second insulation layer 150 is formed on the first insulation layer 120 (covering the first surface 110a of the substrate body 110) and on portions of the first and second conductive layers 130, 140. The second insulation layer 150 includes an opening part 150x. A portion of the second conductive layer 140 is exposed at a bottom part of the opening part 150x. 
The third insulation layer 160 is formed on the first insulation layer 120 (covering the second surface 110b of the substrate body 110) and on portions of the first and second conductive layers 130, 140. The third insulation layer 160 includes an opening part 160x. A portion of the second conductive layer 140 is exposed at a bottom part of the opening part 160x. 
The external connection terminal 170 is formed on the second conductive layer 140 having a portion exposed at the bottom part of the opening part 150x. The first external connection terminal 170 is electrically connected to the second conductive layer 140. The second external connection terminal 180 is formed on the second conductive layer 140 having a portion exposed at the bottom part of the opening part 160x. The second external connection terminal 180 is electrically connected to the second conductive layer 140.
FIGS. 2 through 4 illustrate processes for manufacturing a wiring substrate according to a related art example. First, the trench 110x and the through-hole 110y communicating with the trench 110x are formed in the substrate body 110. Then, the first insulation layer 120 is formed on the first surface 110a of the substrate body 110, the inner bottom surface and the inner side surface of the trench 110x, and the inner side surface of the through-hole 110y. Then, a first conductive layer 130S is formed on the first insulation layer 120 covering the first surface 110a of the substrate body 110, the inner bottom surface and the inner side surface of the trench 110x, and the inner side surface of the through-hole 110y. The first conductive layer 130S is formed by using, for example, a sputtering method. As described below, the first conductive layer 130S is subsequently formed into the first conductive layer 130 after unnecessary portions are removed therefrom.
Then, in the process illustrated in FIG. 3, a copper plate 220 is mounted on the first insulation layer 120 (covering the second surface 110b of the substrate body 110) via an adhesive layer 210. The adhesive layer 210 includes an opening part 210x that is formed in correspondence with the through-hole 110y. Then, a second conductive layer 1405 fills the inside of the trench 110x and the through-hole 110y and covers the first conductive layer 130S. The second conductive layer 140S is formed by using, for example, an electroplating method where the copper plate 220 and the first conductive layer 130S are used as a feeding layer. As described below, the second conductive layer 140S is subsequently formed into the second conductive layer 140 after unnecessary portions are removed therefrom.
Then, in the process illustrated in FIG. 4, the second conductive layer 1405 formed on the first surface 110a of the substrate body 110 is polished by using, for example, a CMP (Chemical Mechanical Polishing) method. Thereby, the second conductive layer 140 is formed along with exposing the first conductive layer 130S on the first surface 110a of the substrate body 110. Then, exposed portions of the first conductive layer 130S are removed by using, for example, an etching method. Thereby, the first conductive layer 130 is formed. Then, the conductive layer 210 and the copper plate 220 illustrated in FIG. 3 are removed.
Then, the second insulation layer 150, the third insulation layer 160, the first external connection terminal 170, and the second external connection terminal 180 are formed in the configuration illustrated in FIG. 4 by using known methods. Thereby, the manufacturing of the wiring substrate 100 illustrated in FIG. 1 is completed.
As described above, the second conductive layer 140S is formed by using an electroplating method where the copper plate 220 and the first conductive layer 130S are used as a feeding layer. In this process of forming the second conductive layer 140S, a plating film simultaneously grows from the first conductive layer 130S at the inner side surface of the trench 110x and from the first conductive layer 130S at the inner bottom surface of the trench 110x. Further, a plating film simultaneously grows from the first conductive layer 130S at the inner side surface of the through-hole 110y and from a portion of the copper plate 220 corresponding to the through-hole 110y. 
Accordingly, in the second conductive layer 140S formed in the trench 110x, defects (e.g., seams, voids) may be formed at the joining portions among the plating films growing from multiple directions. Further, in the second conductive layer 140S formed in the through-hole 110y, defects (e.g., seams, voids) may also be formed at the joining portions among the plating films growing from multiple directions. Such defects (e.g., seams, voids) tend to be formed particularly where the aspect ratio of the trench 110x or the through-hole 110y becomes large. In a case where such defects (e.g., seams, voids) are formed in the second conductive layer 140S, thermal stress may cause disconnection at the second conductive layer or degrading of connecting reliability with respect to the first external connection terminal 170 or the second external connection terminal 180.